Serials like this are a good example as to why the Doctor may sometimes want to listen to people. Ace hears some screaming, the Doctor doesn’t, and Ace turns out to be right.
Besides, this circus sucks.
There’s a good reason it sucks. Visitors don’t go there to see the show. They become part of the show. And the Doctor is called down to join the ringmaster only minutes after taking his seat next to Ace. The only other people in the stands are a bland family of three.
And it turns out those three decide if the acts are any good or not. If they are, the performer gets to live. That’s what Mags was screaming about as she watched someone die. Ace heard that, but she grabbed a dead girl’s earring and was using it as a button. Said button has an eyeball on it, allowing her to be…somewhat invisible to everyone, so she can sneak off and try to do…something.
The Doctor, meanwhile, realizes something is up when he walks into an area of the big top, sees Captain Cook and Mags there, and then the cage door slams shut. For someone as smart as the Doctor normally is, he sure missed the way the crowd sounded much larger than it actually was and a few other clues.
So, with Ace running around, pursued by mostly mechanical clowns, and the fortune-teller selling tickets pining for the days when everything was just fun while the ringmaster talks about power, plus some nerdy fanboy enters the circus willingly while knowing full well what’s going on, there may be some problems.
Cook isn’t helpful. Mags wants out. Cook says Mags has a secret, but in the end, she and the Doctor trick a clown or two into opening the door long enough for them to club the clowns with some juggling clubs and go for a look around. Cook stays because he’s that stubborn sort of character. The Doctor instead finds a lot of what looks like ancient ruins and a nearly bottomless pit under the tent. Oh, wait, there is a giant eye at the bottom of the pit.
Ace, on the other hand, is caught and locked up in the robo-clown repair cart, and some of those things seem to be moving.
And then the clowns and Cook show up to tell the Doctor he’s on next.
Well, maybe the bland family will be entertained by the Doctor’s spoon-playing technique.